Sodium perborate monohydrate with an active oxygen content of approximately 15 to 16% by weight is becoming increasingly more significant than sodium perborate tetrahydrate as a bleaching component in detergents and cleaning agents on account of its higher active oxygen content and higher rate of dissolution.
Sodium perborate monohydrate can be produced, as has already been taught by French patent 1,081,421, by dehydrating sodium perborate tetrahydrate by means of a heated current of air while maintaining the material to be dehydrated in a fluidized state. A disadvantage of this method is the low abrasion resistance of the product produced, which low abrasion resistance results in problems in handling, especially in pneumatic transport.
Attempts have been made to adjust the conditions during the dehydration in an effort to obtain monohydrate with an elevated abrasion resistance.
According to the method described in DE-OS 19 30 286, heated air with a temperature of 180.degree. to 210.degree. C. is supplied to the fluid-bed dryer and the exhaust air temperature maintained at a constant value. The high temperatures used in the DE-OS 19 30 286 process create problems such as the formation of agglomerates and a loss of active oxygen. In addition, the abrasion resistance of the monohydrate obtained in the DE-OS 19 30 286 process no longer meets the current requirements being made in the industry.
DE patent 22 58 319 teaches an improved dehydration method wherein a relative humidity of 40 to 80% is maintained in the air surrounding the crystalline grains and the temperature of the exhaust air is adjusted to at least 60.degree. C. in order that a melting start is achieved. The required moisture is adjusted by feeding in vapor (e.g., steam) to the-drying air. Alternatively, according to patent of addition DE 24 44 780 the relative air humidity can also be produced by evaporating water of crystallization, which, however, considerably lengthens the time for the dehydration, so that the process takes longer and the yield decreases for a given time period.
According to the discussion in EP-B 0,194,952, the two-stage design and the formation and the discharge of dust in the two previously mentioned methods are disadvantageous. It is asserted in, the EP-B 0,194,952 reference that instead of the two-step design it is advantageous to use a one-stage method with an air entry temperature of 100.degree. to 180.degree. C., a fluid-bed temperature of 65.degree. to 80.degree. C. and a relative air humidity in the fluid bed of 10 to 40%. For the dehydration of tetrahydrate on a production scale, the method of EP-B 0,194,952 is limited to the use of a cylindrical fluid-bed dryer. Because of the low admissible flow speed of the drying air and the low admissible moisture associated with this type of dryer, only a low speed, large volume mass current of drying air (and the drying energy provided by the drying air) can be charged into the fluid bed. Hence, only a low production throughput is possible with a system like that of EP-B 0,194,952.